1. Field of the Invention
The invention pertains to the field of variable cam timing systems. More particularly, the invention pertains to an apparatus for allowing actuation of a phaser during low cam torsionals.
2. Description of Related Art
Internal combustion engines have employed various mechanisms to vary the angle between the camshaft and the crankshaft for improved engine performance or reduced emissions. The majority of these variable camshaft timing (VCT) mechanisms use one or more “vane phasers” on the engine camshaft (or camshafts, in a multiple-camshaft engine). In most cases, the phasers have a housing with one or more vanes, mounted to the end of the camshaft, surrounded by a housing with the vane chambers into which the vanes fit. It is possible to have the vanes mounted to the housing, and the chambers in the housing, as well. The housing's outer circumference forms the sprocket, pulley or gear accepting drive force through a chain, belt or gears, usually from the camshaft, or possibly from another camshaft in a multiple-cam engine.
Two types of phasers are Cam Torque Actuated (CTA) and Oil Pressure Actuated (OPA). In OPA or torsion assist (TA) phasers, the engine oil pressure is applied to one side of the vane or the other, in the retard or advance chamber, to move the vane. Motion of the vane due to forward torque effects is permitted.
In a CTA phaser, the variable cam timing system uses torque reversals in the camshaft caused by the forces of opening and closing engine valves to move the vane. Control valves are present to allow fluid flow from chamber to chamber causing the vane to move, or to stop the flow of oil, locking the vane in position. The CTA phaser has oil input to make up for losses due to leakage but does not use engine oil pressure to move the phaser. CTA phasers have shown that they provide fast response and low oil usage, reducing fuel consumption and emissions. However, in some engines, i.e. 4 cylinder, the torsional energy from the camshaft is not sufficient to actuate the phaser over the entire speed range of the engine, especially the speed range where the rpm is high.
FIG. 7 shows a graph of actuation rate versus rpm. When the revolutions per minute (rpm) is low, cam torsional energy is high. When rpm is high, cam torsional energy drops off. The actuation rate for an oil pressure actuated (OPA) or torsion assist (TA) phaser is shown by the dashed line. Since oil pressure is low at low rpm, the actuation rate is also low. As the rpm increases, the oil pressure increases and the actuation rate of the OPA or TA phaser also increases. The solid line shows the actuation rate of the cam torque actuated (CTA) phaser. The CTA phaser is actuated by torsional energy, which is high at low rpm and low and higher rpm.
Numerous strategies have been used to solve the problem of low cam torsional energy at high rpm or high engine speeds. For example, if the position of the cam phaser was to full retard during the periods of low torsional energy, the friction of the cam drive may be used to pull the phaser back to the full retard position. Another strategy is to add a bias spring to help move and hold the phaser to a full advance position during periods of low torsional energy. Other examples are shown in U.S. Pat. Nos. 6,276,321, 6,591,799, 5,657,725, and 6,453,859.
U.S. Pat. No. 6,276,321 uses a spring attached to a cover plate to move the rotor to an advanced or retard position to enable a locking pin to slide into place during low engine speeds and oil pressure.
U.S. Pat. No. 6,591,799 discloses a valve timing control device that includes a biasing means for biasing the camshaft in an advanced direction, where the biasing force is approximately equal to or smaller than a peak value of frictional torque produced between a cam and a tappet.
U.S. Pat. No. 5,657,725 discloses a CTA phaser that supplies full pressure to an ancillary vane that provides bias to the phaser based on the pressure of the oil pump. The oil pressure bias uses an open pressure port and lacks proportional control at high engine speeds.
U.S. Pat. No. 6,453,859 discloses a single spool valve controlling a phaser having both a CTA and two check valve torsional assist (TA) properties. A valve switch function is used to switch from CTA to TA during periods of low torsional energy.